US2553937A - Method of electrolytically polishing aluminum and its alloys - Google Patents

Description

atented May 22, 1951" UNITED STATES PATENT OFFICE lllETHOD OF ELECTROLYTICALLY POLISH- ING ALUMINUM AND ITS ALLOYS No Drawing. Application January 11, 1949, Serial No. 70,364. In France January 17, 1048 4 Claims. (01. 204-1405) It is known to produce surfaces having a high reflective capacity on aluminum and aluminum allows by the use of electrolytic burnishing or polishing methods. In such methods the part to be burnished is used as an anode in a successive series of electrolytic baths; one of those baths is called a, bumishing bath and produces the burnishing eiiect proper, with or without a polishing action; a subsequent bath may be called a .re-

nforcing bath and serves to protect the burfiiishcd metal with a transparent film of oxide.

. The steps in the treatment may for example include: 1) Thorough mechanical bufling;

(2) Cleaning with a solvent or by electrolysis;

(3) Electrolytic burnishing, during which a first film of oxide is formed;

(4) Dissolving said initial oxide film in a phospho-chromic mixture.

(5) Washing in tap-water.

(6) Reenforcement in any one of the usual oxidising baths.

(7) Rinsing in tap-water.

(8) Dyeing.

(9) Filling-in the pit-holes.

(10) Brightening.

My present invention is more especially concerned with the electrolytic burnishing step. While a number of suitable baths are already known some of which are employed on a commercial scale, in practice they all show serious shortcomings. Thus some of the known baths are only satisfactory when operated in connection with high-purity aluminum, whereas others on the other hand are successful only when used on aluminum alloys; inmost cases moreover the operating temperature-of known baths is high, being close to 100 C. Another drawback of a number of known baths is that the current density usable therein is too low to permit a rapid burnishing process. Then again, they may require a thorough mechanical bufiing treatment prior to the electrolysis treatment. Another frequent drawback is that only one face of the part to be burnished may be treated at a time or defects may arise as a result of the treatment, due for instance to a regular discharge of gases. Finally, known baths may comprise rare and expensive constituent materials, or substances danerous to handle.

I have now discovered that through the use of certain classes of electrolytic baths, part or all of the above shortcomings of prior baths may be eliminated and a more convenient and simplified electrolytic .burnishing treatment provided for .2 industrial use on aluminum and aluminum containing alloys.

According to the invention, there is provided an electrolyte for electrolytic burnishing of aluminum and its alloys, which comprises as main constituents nitric acid and water. The bath may contain only nitric acid and water. Or alternatively I have obtained satisfactory results by adding to the nitric bath an acid or a mixture of acids having a high alumina-dissolving capacity. Thus excellent results have been obtained when adding horic acid to the nitric bath. Still more sat1sfac u a e a I t adding to the nitric bath a mixture of phosphoric and chromic acids.

I have obtained a handsome polish and burnishing effect when maintaining the operative conditions within the following ranges:

(1) the nitric acid content within an approximate range from 5% to when a nitric bath is used without any added acid or mixed acids:

(2) when only phosphoric acid is added to the nitric bath, the following approximate ranges of compositions are preferably maintained:

(3) When a mixture of phosphoric acid and chromic acid is added to the nitric bath, the approximate preferred ranges are as follows:

Nitric acid from 35% to 45% Phosphoric acid from 8% to 24% Chromic acid from 3% to 10% Water as required to make In the above tabulations and those to follow the percentages are by weight and the acids used are assumed to be anhydrous. In practice we may use:

Nitric acid at 36 Baum, specific gravity d=1.332, containing 52% by weight of nitric acid.

Nitric acid at 40 B., specific gravity d=1.383, Con ining by weight of nitric acid; this type of acid is used where the bath contains over 52% nitric acid.

Nitric acid at 42 B.. specific gravity d=1.41, containing 92.76% by weight nitric acid; this is used where the bath contains over 62% nitric acid. ,m.

Phosphoric acid-"'at 59 l3., specific gravity d=1.691, containing .Bflfhby weight phosphoric acid.

According to aspects of the invention, the following preferred operative conditions are em- 3 ployed when using the improved electrolysis bath:

(1) the bath is operated at surrounding temperature. Preferably the temperature is mamtained under 20 C.

(2) A high degree of agitation is necessary to obtain good results. Thus insumcient agitation or non-uniform agitation will produce on the aluminum surfaces reflection-distorting fleece-like eflects.

(3) The current density may be varied in the range of from about 90 to about 450 amperes per square foot with a corresponding time range of from about to about one or two minutes, according to the current density and the initial surface condition of the articles.

The cathode used in the improved bath is an aluminum or a graphite cathode. The tanks or containers containing the bath may be made of stainless steel, various plastics, ebonite-coated iron or generally any suitable materials impervious to the 'acids used in my process. Coolin means should be provided, since heat is rapidly evolved in the bath and, as stated, the temperature thereof should not exceed about C. in operation.

The steps prior to and the steps subsequent to the bumishing step may be accomplished in any of the well-known ways. Thus for the solventcleaning operation, a substance such as trichloroethylene or the like may be used. If the cleaning operation is to be carried out by electrolysis, I may use, for example, a, bath of the following composition:

Grams Anhydrous trisodium phosphate 100 Anhydrous sodium carbonate 200 Soda silica 5 This bath is used at (about 20 C. temperature 'with direct current. The articles to be cleaned Phosphoric acid: 360 cc. of an 87% HaPO4 solution (d=1.691)

Chromic acid: 180 grams Water: 640 cc.

The reenforcing treatment may comprise an anodic oxidisation in an aqueous bath containing 20% of its weightof 36 B. sulphuric acid. This bath may be used at a temperature of about 20 C. with a current density of about 13.8

amperes/sq. it.

Some examples will now be given of typical bath compositions according to the invention to- Egther with the conditions used in operating The time of treatment is from about 10 to 15 minutes, during which strong agitation is maintained.

The bath of this example is particularly suit able for high purity aluminum.

Example 2.Nitric acid: 62%

Water: 38% Conditions of operation:

Temperature: 20 C. Voltage: 4 v. Current density: 280 amp/sq. it.

The time of treatment is from about 10 to 15 minutes, with strong agitation throughout.

The bath of this example is especially suitable for high-purity aluminum.

Example 3.-Nitric acid: 41.5%

Phosphoric acid: 17.5% Water: 41.0% Conditions of operation:

Temperature: 10 to 20 C. Voltage: 3 to 7 v. Current density: 186 to 370 amp/sq. ft.

Example 4.Nitric acid: 41%

Phosphoric acid: 14 Chromic acid: 5 Water: 40 Conditions of operation:

Temperature: 10 to 20 C.

Voltage: 3 to '7 v.

Current density: 186 to 370 amp/sq. ft.

(1) The articles to be burnished do not require to be subjected prior to burnishing to a thorough mechanical buffing. since the bath is effective to produce in addition to burnishing, a slight polishing or buffing effect. It is merely necessary to effect a rough mechanical polish in order to remove the coarser asperities from the surface, such as scratches and surface irregularities found in metal castings direct from the foundry.

(2) The improved bath burnishes at room temperature. The fact that it is operative under 20 C. forms one of the most remarkable characteristics of the present invention. Thus it will not be required to provide, as in the conventionally used methods, heating means for the bath container, and this will substantially reduce cost of the plant installation. Moreover the cost of operation 01. the process is also substantially lowered since it is no longer necessary to hold the bath at high temperature.

(3) The articles treated are polished and burnished on both faces and their appearance is not marred by the presence of stripes due to regular discharge of gases. This advantage is particularly appreciable in connection with the manufacture ofarticles used for ornamental purposes.

(4) Use of my improved bath makes it possible to operate under a very low voltage (about 3 to 7 volts) whereby the item relating to electric energy consumption in the over-all cost price will be notably diminished.

The substances used in the composition of the bath are widely available, and may be handled without any exceptional precautions.

(6) The bath is applicable for burnishing both aluminum of high purity and of ordinary grades, as well as aluminum base alloys.

Among the aluminum base alloys in connection with which my improved electrolytic burnishing bath is applicable, I may mention aluminum alloys containing copper (such as duralumin) magnesium (such as aluminum alloys containing from 3 to 5% magnesium), magnesium and silicon containing alloys such as almasilium, and aluminum alloys containing magnesium and zinc. A handsome burnish may also be obtained on aluminum alloys of the moldable type, such as those containing from 3 to 5% magnesium, the alloy known as A. P. M. (4.2% Cu, 0.184140% Mg, (LN-0.35% Ti), the alloy known as Y (35-45% Cu, l.2-1.8% Mg, 1.7-2.3% Ni) and others.

What I claim is:

1. In a method for electrolytically polishing and brightening articles consisting at least for the major part of aluminum, the step of effecting the electrolytic polishing and brightening treatment at a temperature below 20 C. in an electrolysis bath containing 30 to 45% by weight of nitric acid and a significant amount of at least one member of the group consisting of phosphoric and chro'mic acids, the total of the acids ranging between 43 and 79% by weight, the balance being water.

2. In a method for electrolytically polishing and brightening articles consisting at least for the major part of aluminum, the step of effecting the electrolytic polishing and brightening treatment under a current tension of 3 to 7 volts and at a current density of 10 to 50 amperes per square decimeter, and at a temperature below 20 C. in an electrolysis bath containing to 45% by weight of nitric acid and a significant amount of at least one member of the group consistin of phosphoric and chromic acids, the total of the acids ranging between 43 and 79% by weight, the balance being water.

3. In a method for electrolytically polishing and brightening articles consisting at least for the major part of aluminum, the step of effecting the electrolytic polishing and brightening treatment at a temperature below 20 C. in an electrolysis bath containing 30 to 45% by weight of nitric acid and 13 to 34% by weight of phosphoric acid, the balance being water.

4. In a method for electrolytically polishing and brightening articles consisting atleast for the major part of aluminum, the step of effecting the polishing and brightening treatment at a temperature below 20 C. in an electrolysis bath containing 35 to by weight of nitric acid, 8 to 24% by weight of phosphoric acid, and 3 to 10% by weight of chromic acid, the balance being water.

J OS PATRIE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,462,197 Jernstedt Feb. 22, 1949 FOREIGN PATENTS Number Country Date 504,026 Great Britain Apr. 17, 1939 798,721 France Mar. 11, 1936

Claims (1)

1. IN A METHOD FOR ELECTROLYTICALLY POLISHING AND BRIGHTENING ARTICLES CONSISTING AT LEAST FOR THE MAJOR PART OF ALUMINUM, THE STEP OF EFFECTING THE ELECTROLYTIC POLISHING AND BRIGHTENING TREATMENT AT A TEMPERATURE BELOW 20* C. IN AN ELECTROLYSIS BATH CONTAINING 30 TO 45% BY WEIGHT OF NITRIC ACID AND A SIGNIFICANT AMOUNT OF AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF PHOSPHORIC AND CHROMIC ACIDS, THE TOTAL OF THE ACIDS RANGING BETWEEN 43 AND 79% BY WEIGHT, THE BARLANCE BEING WATER.